What is the specific good news that Jesus preached to "repent and believe"?

You have skipped that part of His message. So here is the full passage in New English Translation first, then in KJV to provide the translation most used by 2x2s.

Mark NET 1:14 Now after John was imprisoned,29 Jesus went into Galilee and proclaimed the gospel30 of God.31 1:15 He32 said, “The time is fulfilled and the kingdom of God33 is near. Repent and believe the gospel!”

“the gospel of God,” occurs nowhere else in the Gospels, while “the gospel of the kingdom” is a Matthean expression (Matt 4:23; 9:35; 24:14), and “kingdom of God” is pervasive in the synoptic Gospels (occurring over 50 times).

Mark KJV 14 Now after that John was put in prison, Jesus came into Galilee, preaching the gospel of the kingdom of God, 15 And saying, The time is fulfilled, and the kingdom of God is at hand: repent ye, and believe the gospel.

Full Translators notes for the NET version

29tn Or “arrested,” “taken into custody” (see L&N 37.12).

30tc Most witnesses, especially later ones (A D W Ï lat), have τῆς βασιλείας (ths basileias) between τὸ εὐαγγέλιον (to euangelion) and τοῦ θεοῦ (tou qeou): “the gospel of the kingdom of God.” On the one hand, it is perhaps possible that τῆς βασιλείας was omitted to conform the expression to that which is found in the epistles (cf. Rom 1:1; 15:16; 2 Cor 11:7; 1 Thess 2:2, 8, 9; 1 Pet 4:17). On the other hand, this expression, “the gospel of God,” occurs nowhere else in the Gospels, while “the gospel of the kingdom” is a Matthean expression (Matt 4:23; 9:35; 24:14), and “kingdom of God” is pervasive in the synoptic Gospels (occurring over 50 times). Scribes would thus be more prone to add τῆς βασιλείας than to omit it. Further, the external support for the shorter reading (א B L Θ Ë1,13 28* 33 565 579 892 2427 sa) is significantly stronger than that for the longer reading. There is little doubt, therefore, that the shorter reading is authentic.

31tn The genitive in the phrase τὸ εὐαγγέλιον τοῦ θεοῦ (to euangelion tou qeou, “the gospel of God”) could be translated as either a subjective genitive (“the gospel which God brings”) or an objective genitive (“the gospel about God”). Either is grammatically possible. This is possibly an instance of a plenary genitive (see ExSyn 119-21; M. Zerwick, Biblical Greek, §§36-39). If so, an interplay between the two concepts is intended: The gospel which God brings is in fact the gospel about himself.

32tn Because of the length and complexity of the Greek sentence, a new sentence was started here in the translation.

33sn The kingdom of God is a reference to the sovereign activity of God as he rules over his creation and brings his plans to realization.